1. Field of the Invention
The present invention relates to an image processing method and apparatus and, more particularly, to an image processing method and apparatus for performing pseudo-halftoning for multi-valued input data.
2. Description of the Related Art
A conventional image processing apparatus such as an electrophotographic printer generates a binary signal representing the ON/OFF state of a dot on print paper, i.e., whether or not to form a dot. The image processing apparatus prints an image in accordance with the binary signal. The tone of an image is expressed by the number of dots formed per unit area. Thus, binarization processing is generally called pseudo-halftoning.
Pseudo-halftoning which determines the ON/OFF state of a dot uses a halftone screen or halftone dot. Halftone screen processing in a digital image processing apparatus uses a two-dimensional threshold matrix to execute binarization processing for determining the ON/OFF state of a dot. More specifically, each threshold in the threshold matrix is compared with a corresponding pixel value of an input multi-valued image. A value equal to or larger than the threshold or a value smaller than it is expressed by a binary value {0 or 1}.
There have conventionally been proposed several techniques for improving a jaggy generated in a low-resolution printer and the like. The jaggy is roughness (aliasing) of the edge portion of a character image or the like. FIG. 11 shows an example of the jaggy. FIG. 11 shows a plurality of squares, and each square corresponds to one pixel. FIG. 11 shows an example at 300 dpi. When the image resolution is as low as 300 dpi, as shown in FIG. 11, particularly an oblique edge portion is jagged.
A conventional technique improves such a jaggy by adding using pattern matching image data to a portion which matches a pattern, or removing a pixel causing the jaggy. Details of pattern matching are well known, so a detailed description thereof will be omitted. Japanese Patent Laid-Open No. 4-341060 discloses an example of a smoothing technique.
FIGS. 12A and 12B show examples of improving the jaggy. FIG. 12A shows an example of adding halftone data of low density represented by hatched portions in FIG. 12A to the peripheries of pixels matched by pattern matching. Halftone data is added to a portion where a jaggy occurs, visually suppressing generation of the jaggy.
FIG. 12B shows an example of further dividing pixels matched by pattern matching. Pixel division is a technique of dividing one pixel at 300 dpi in the above-mentioned example into a plurality of partial areas, and determining the ON/OFF state of a dot in each partial area. The pixel division technique is also well known (e.g., Japanese Patent Laid-Open No. 9-321986), and a detailed description thereof will be omitted.
However, these methods still leave the following problem unsolved.
When pseudo-halftoning, e.g., screen processing is done for a character image of low density (or low brightness), as shown in FIG. 13A, it may lead to a printed result as shown in FIG. 13B. That is, a low-density character image undergoes pseudo-halftoning at a screen resolution lower than the 300-dpi printer resolution. As a result, the character or thin line becomes disconnected. This is because the screen resolution is pseudo-represented at once for a plurality of pixels (dots) of the printer, so pseudo-halftoning is not executed using a screen resolution higher than the printer resolution. For example, a general screen resolution (screen ruling) in the printing industry is 85 to 175 lines, and about 268 lines at most. At a high LPI (Lines Per Inch) of 200 or more, an electrophotographic printer hardly obtains a stable image quality owing to mechanical characteristics. Owing to the relationship between the printer resolution and the screen ruling, a halftone character or thin line having undergone pseudo-halftoning becomes disconnected. Thus, smoothing processing by the above-described pattern matching cannot suppress a jaggy and disconnection, and there is still room for improvement.
FIG. 14 macroscopically shows part of a character image. As is apparent from FIG. 14, an undesirable jaggy sometimes appears at the edge portion of an image obtained by performing screen processing for a halftone image. In this way, a jaggy generated at low LPI in screen processing still remains unsolved.